The present invention concerns a receiver designed to pick up an electromagnetic signal coming from a transmitter. It also concerns a system for tracking objects, incorporating such a receiver.
In the state of the art, these receivers generally comprise:
a passive receiving antenna;
a resonant circuit connected to the antenna, formed of passive components and replying, by transmitting an electric signal, to the electromagnetic signal picked up, and
an electric signal processing circuit.
If such receivers are carried by a person and have to be constantly in listening mode, the source of electric energy which powers them has to be fairly large, failing which it is likely to be rapidly exhausted, because the known processing circuits consume a great deal of energy. These receivers can therefore be miniaturized only by reducing their self-sufficiency.
To reduce this consumption, document WO-00 28 475 includes an amplifier and a circuit for integration of the signals received, placed between the resonant circuit and the threshold comparator circuit. In this way, the processing circuit is activated only when the load integrated exceeds a limiting value. This procedure has two drawbacks, however. First, if the receiver is in the vicinity of the source, in an area where the electromagnetic signal slightly exceeds the background noise level, the threshold comparator circuit can be activated, whereas the level of the signal is inadequate to identify the transmitter in the vicinity of which the receiver is located. This, therefore, results in unwanted triggerings which increase the receiver""s consumption. Secondly, if the receiver passes very quickly the transmitter, it is possible that the integrated load level may not be sufficient to activate the threshold comparator circuit.
In this way, the passage is not recorded. In other words, the reactions of the threshold circuit vary depending on the position and speed of the receiver. This greatly limits the conditions of use.
Similar solutions are described in documents EP 0440 153 and GB 2 208 058. In both of these documents, an integration circuit is placed between the resonant circuit and the threshold detector circuit, and the same problems are accordingly observed.
The purpose of the present invention is to counter these disadvantages. This is achieved due to the fact that the receiver in accordance with the invention, as described above, comprises a threshold comparator circuit, defining a reference threshold and connected directly to the terminals of the resonant circuit to compare the electric signal, not amplified, with the reference threshold and activate the processing circuit only when the amplitude of the signal exceeds that threshold. In other words, as soon asxe2x80x94and only whenxe2x80x94the voltage of the signal picked up by the antenna and measured across the terminals of the resonant circuit exceeds the threshold value is the comparator circuit activated.
It is strongly recommended that the electromagnetic signal have a frequency in the range between 1 kHz and 1 MHz and that the natural frequency of the resonant circuit, formed of a capacitor and a coil connected in parallel, comply with the frequency of this signal.
The present invention has an especially interesting application in systems for tracking objects moving within a given space, especially for monitoring people in homes or hospitals. In this case, it is essential that the carried module may not hinder the person making use of it, and that its lifetime be as long as possible.
To this effect, the object tracking system according to the invention comprises:
a set of RF beacon transmitters, distributed throughout the space in question and fitted so as to each transmit an electromagnetic signal carrying a beacon identification code which is specific to it;
linked to each of the objects to track, a RF transmitter-receiver module including:
a RF beacon receiver to pick up the electromagnetic signal transmitted by the beacon to which it is closest and comprising a passive receiving antenna, a resonant circuit connected to the antenna, formed of passive components and replying, by transmitting an electric signal, to the electromagnetic signal picked up, and a threshold comparator circuit defining a reference threshold and connected directly to the terminals of the resonant circuit to compare the electric signal, not amplified, with the reference threshold and to deliver an activation signal when the amplitude of the electric signal exceeds the threshold, indicating that the object is in the vicinity of a beacon;
a processing circuit containing in memory an object identification code and replying to the activation signal by placing in memory the code of the beacon in the vicinity of which the object is located, and
an RF identification transmitter to transmit a signal carrying the module identification code and the stored beacon identification code;
at least one RF identification receiver to receive the signals transmitted by the module linked to the objects, and
a base station for processing the data received by the identification receiver.
Preferably, each module shall also comprise means of triggering transmission of the carrier signal either at the request of the person carrying the object, or automatically whenever the memory content is altered.
In some applications, for example when the antenna of the RF beacon transmitter must be located far from a source of energy, when it is in an open space, or again when two of them are close to one another, it is worthwhile having the system equipped with a RF beacon transmitter which includes a resonant circuit consisting of:
a coil provided with two terminals one of which is connected to earth;
a capacitor formed of a conducting plate placed on the ground in an area of the space in question and of said earth, and
a single conducting wire linking said plate to the other terminal of the coil.